Q&A: Effectiveness of Thermal Curtains

Q.Do so-called
“double-cell” insulating window
shades perform as advertised? One salesperson claims
R-3 if they are installed with an airtight edge seal.
But what if they are installed without the perimeter
track?

A.Paul Fisette, director of
Building Materials and Wood Technology at the
University of Massachusetts Amherst and a JLC
contributing editor, responds: I
can’t tell you whether your curtains are
truly R-3, though that does not seem like an
unreasonable R-value if they are installed with
tracks. I have seen some shades advertised that
claim values as high as R-8, which I’m
skeptical about. You’ll definitely get the
best performance and are more likely to get the
full reported R-value from the shades if you use an
airtight seal around the edge of the window.
Without the seal and with the shades drawn, you
will most likely find condensation around the edges
of the glazing — an indication that warm
air from the room is leaking in through the gaps
between the shade and the window trim and reaching
the cold condensing surface of the glass.

This is evidence that the shades are in fact
insulating your windows to some degree and
providing some energy benefit, because the window
glass is colder than room temperature. Say, for
example, that the room temperature is
70°F and the relative humidity is
50 percent; in that case, the dew-point temperature
is around 49°F. If it’s a
freezing night and you pull the shade, thereby
preventing warm room air from contacting the glass,
it may not take long for the glass to reach
dew-point temperature. You’ll get
condensation around the edges, where heat is lost
to the window frame; with double-hungs,
you’ll probably also have condensation
just above the meeting rails, where air leaks are
common. But if you leave the shade raised, the
glass will stay in contact with the room air and
remain warmer. You won’t get condensation,
but you’ll be expending additional heating
energy. So condensation is actually a sign that the
shades are working to some degree — though
the wetting is not good for the long-term
durability of the windows. Therefore I would go the
extra mile and install the edge seals.

One last note: In my climate (around 6,800
heating-degree days), you save approximately 2
percent on your heating bill for every degree you
reduce your thermostat on a 24-hour basis. Because
warmer surfaces radiate heat to colder surfaces,
using thermal curtains can make a room feel more
comfortable at a lower temperature.